Motion Blur Issues With License Plate Recognition
Shutter speed & light the keys to LPR applications
We are using a standard CCTV camera for LPR – the camera has the software capability, but we are finding results patchy. An issue is motion blur, even at relatively slow speeds. The camera’s default low light shutter setting is 1/25th of a second and we have it at 100th of a second, which does make the scene somewhat darker. Could you give us your thoughts on appropriate setup?
There are lots of issues to take into account with license plate recognition. Just off the cuff we’d be looking at a longer lens – a prime if possible (though we don’t know if a 5-6mm fixed focal length CCTV lens exists). This will give you maximum aperture at a slightly longer focal length, increasing sharpness and removing unwanted angle of view.
You’ll need to consider angle of incidence between lens and license plate. You don’t want it to be too acute, or performance will be weakened. To reduce angle of incidence, position the camera on secure side of the gate – within reason, the further from the plate the camera is, the less the angle of incidence.
We’d be thinking about a directional light source that need not be enormously bright, but must be capable of illuminating the front of the vehicle as it approaches entry and exit points. You need to think about a light source capable of generating reflectance from the different coloured surfaces it will encounter, including red.
A capable light source will circumvent many of your issues. But before you go down that path, try a shutter speed of 250th of a second. You will need that sort of shutter speed to ensure a plate at around 10kmph. A somewhat darker scene is acceptable if you always get a plate and a plate is only what you need.